Viability Beyond Performance: Regime-Level Coupling and Structural Cost in Long-Horizon Adaptive Systems

This work provides a conceptual and architectural analysis of long-horizon viability in adaptive systems, focusing on failure modes that remain invisible to standard performance-based evaluation. It argues that correctness of behaviour, bounded error, or sustained reward are insufficient indicators of long-term robustness when corrective intervention becomes structurally inefficient. The paper introduces a regime-level framing of viability that separates outward behavioural admissibility from long-horizon structural sustainability. Central to this framing is the distinction between inertial and intervention-dense operational regimes, and the role of coupling efficiency in determining the effective structural cost of corrective action. Without prescribing algorithms, metrics, or implementation details, the work articulates non-algorithmic architectural relations linking intervention intensity, coupling efficiency, and viability. These relations are intended as qualitative and relational heuristics that capture structural dependencies governing long-horizon sustainability, rather than as optimisation objectives or control laws. The contribution is positioned as conceptual prior art. It fixes scope, terminology, and architectural constraints that any adaptive system claiming long-horizon robustness must account for, independent of specific mechanisms, learning paradigms, or embodiments. The framework applies broadly across control systems, learning-based agents, embodied systems, and hybrid architectures operating under uncertainty, non-stationarity, and cumulative stress.